This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-398865, filed Dec. 27, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to color cathode ray tube apparatuses for TV sets, monitors, etc., and more particularly, to a color cathode ray tube apparatus capable of deflecting electron beams at wide angles.
2. Description of the Related Art
Color cathode ray tube apparatuses of the so-called self-convergence/in-line type are currently widely used. One such cathode ray tube apparatus comprises an in-line electron gun assembly, which emits three electron beams in a line, including a center beam and a pair of side beams that pass along one and the same horizontal plane. It further comprises a deflection device that generates a pincushion-type horizontally deflecting magnetic field and a barrel-type vertically deflecting magnetic field. This cathode ray tube apparatus, combining the electron gun assembly and the deflection device, converges the three electron beams on the whole area of a screen without requiring use of any special correcting circuit.
In general, in the color cathode ray tube apparatus of this type, the electron gun assembly emits the side beams at a given angle so as to converge the three electron beams at the center of the screen. The convergence of the three electron beams on the center of the screen is adjusted by means of a purity-convergence magnet (PCM) that is formed of a ring-shaped magnet on a neck portion of a color cathode ray tube.
Conventionally, there is a proposal to improve the convergence characteristics of the three electron beams by means of various types of correcting coils that are arranged in the deflection device. Described in Jpn. Pat. Appln. KOKAI Publication No. 9-265922, for example, is a correcting coil that is attached to the side of an electron gun assembly of a deflection yoke and generates a quadrupole magnetic field, whereby the convergence of the three electron beams can be corrected. Described in Jpn. Pat. Appln. KOKAI Publication No. 10-112272, moreover, is an auxiliary coil that is wound around a core of a deflection device for the same purpose. Described in Jpn. Pat. Appln. KOKAI Publication No. 51-85630, furthermore, is a barrel-type magnetic field, not pincushion-type, for dynamic convergence correction, which is used to correct deflection defocusing of electron beams.
The depth of a prevalent large-screen color cathode ray tube apparatus is increased in proportion to its screen size. If the screen size is enlarged with the maximum deflection angle of electron beams fixed, a reference point for deflection must be kept away from the screen in order to deflect the electron beams to the whole area of the large screen.
Recently, on the other hand, there has been an increasing demand for large-screen color cathode ray tube apparatuses with reduced depths. The depth of a large-screen cathode ray tube apparatus can be reduced most effectively by enlarging the deflection angle. However, the enlargement of the deflection angle considerably lowers the image quality in the peripheral portion of the screen or causes an increase in necessary dynamic focus voltage.
The lowering of the image quality in the peripheral portion of the screen occurs because deflection defocusing of electron beams which is horizontally extending beam distortion is accelerated as the deflection angle is enlarged. As described in the above, in order to converge the three electron beams also on the peripheral portion of the screen, the deflection device generates a non-uniform magnetic field that is formed of a barrel-type vertically deflecting magnetic field and a pincushion-type horizontally deflecting magnetic field. This non-uniform magnetic field also influences the shape of the electron beams. In particular, deflection defocusing that is caused by the horizontally deflecting magnetic field arouses a problem.
The influence of the horizontally deflecting magnetic field on the electron beams will now be described with reference to FIGS. 6A and 6B. In the description with reference to these drawings, the electron beams are supposed to be deflected to the right-hand side of the screen. As shown in FIG. 6A, the pincushion-type horizontally deflecting magnetic field, by virtue of its shape, generates a force that vertically depresses the electron beams and a force that laterally spreads the electron beams. These forces become stronger as the deflection angle widens or as the horizontally opposite end portions of the screen are approached.
In consequence, a beam spot has a horizontally elongated or oblong shape at each of the horizontally end portions of the screen, as shown in FIG. 6B. Even if a beam spot in the center of the screen is circular, therefore, beam spots at the horizontally end portions of the screen, obtained after the electron beams are horizontally deflected by the horizontally deflecting magnetic field, are oblong, so that the resolution of the image is lowered.
Further, the horizontally extending of the electron beams promotes a moirxc3xa9 effect in the peripheral portion of the screen. The wider the deflection angle, the higher the intensity of the horizontally deflecting magnetic field or pincushion-type magnetic field should be. Thus, the horizontally extending of the electron beams is enhanced with the enlargement of the deflection angle.
If the deflection angle is enlarged, the difference in the electron beam path length between the center and the peripheral portion of the screen increases. The increase in the path length difference causes vertical overfocusing of the electron beams by the horizontally deflecting magnetic field. Thus, the difference in the required proper magnification of the electron gun assembly between at the center and at the peripheral portion of the screen increases inevitably.
Accordingly, there is a great difference in dynamic focus voltage between the case where an electron beam is focused on the center of the screen and the case where the electron beam is focused on the peripheral portion of the screen. Thus, in order to maintain focusing characteristics for the peripheral portion of the screen without ruining focusing characteristics that are allowed for the center of the screen, the dynamic focus voltage must be increased in focusing the electron beams on the periphery of the screen. With use of an ordinary deflection angle (about 110xc2x0), the difference in dynamic focus voltage is adjusted to at most about 1 kV in order to focus the electron beams optimally on the center and the horizontally opposite end portions of the screen. If the deflection angle is wider (about 120xc2x0), on the other hand, the difference in dynamic focus voltage is several kilovolts.
The increase of the dynamic focus voltage constitutes a heavy load on the circuit of a TV set or monitor. If the dynamic focus voltage is too high, moreover, the color cathode ray tube apparatus itself arouses a problem on the withstand voltage. That is, the dynamic focus voltage, along with a dynamic component, its increment, is supplied from stem pins at the neck end portion. The stem pins are supplied with various voltages, such as cathode voltage, heater voltage, focusing voltage, etc., for controlling the color cathode ray tube apparatus. This is done because if the dynamic focus voltage is too high, there is a great voltage difference between the stem pins when a voltage is applied to the pins, so that the limit of the withstand voltage may possibly be exceeded.
In order to converge the three electron beams on the periphery of the screen, in general, moreover, the respective trajectories of the side beams are changed in the electron gun assembly by shifting central axes of the side beam holes between opposite electrodes that constitute a main lens portion, so that the side beams are emitted at a given angle from the electron gun assembly. If the difference in dynamic focus voltage is great, therefore, the side beams horizontally extended to so high a degree that the difference in shape between the center and side beams cannot be ignored.
Thus, in the color cathode ray tube apparatus of the wide-angle deflection type, the electron beams are horizontally extended by the non-uniform deflecting magnetic field of the deflection device, thereby lowering the resolution, and the dynamic focus voltage is increased to arouse a problem on the withstand voltage.
The present invention has been contrived in consideration of these circumstances, and its object is to provide a color cathode ray tube apparatus capable of providing wide angles of deflection of electron beams and high resolution.
A color cathode ray tube apparatus according to an aspect of the present invention comprises: a cathode ray tube having an electron gun assembly which emits three electron beams arranged in a line and a phosphor screen which glows as the electron beams emitted from the electron gun assembly hit the phosphor screen; and a deflection device having a horizontally deflecting coil for generating a horizontally deflecting magnetic field which deflects the electron beams in a horizontal direction and a vertically deflecting coil for generating a vertically deflecting magnetic field which deflects the electron beams in a vertical direction, the electron gun assembly emitting at least one electron beam having an oblong cross-sectional shape extending substantially in the horizontal direction and emitting the three electron beams in a substantially non-convergent state toward the central portion of the phosphor screen, the deflection device including an auxiliary magnetic field generator for generating a quadrupole magnetic field component which focuses the electron beam with an oblong cross section more heavily in the horizontal direction than in the vertical direction, the auxiliary magnetic field generator being located in a given region in a tube-axis direction in which the horizontally deflecting coil is located.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.